feat(ui): add Now Playing panel for admins (#4209)

* feat(ui): add Now Playing panel and integrate now playing count updates

Signed-off-by: Deluan <deluan@navidrome.org>

* fix: check return value in test to satisfy linter

* fix: format React code with prettier

* fix: resolve race condition in play tracker test

* fix: log error when fetching now playing data fails

Signed-off-by: Deluan <deluan@navidrome.org>

* feat(ui): refactor Now Playing panel with new components and error handling

Signed-off-by: Deluan <deluan@navidrome.org>

* fix(ui): adjust padding and height in Now Playing panel for improved layout

Signed-off-by: Deluan <deluan@navidrome.org>

* fix(cache): add automatic cleanup to prevent goroutine leak on cache garbage collection

Signed-off-by: Deluan <deluan@navidrome.org>

---------

Signed-off-by: Deluan <deluan@navidrome.org>

utils/cache/simple_cache.go

@@ -1,8 +1,10 @@
 package cache
 
 import (
+	"context"
 	"errors"
 	"fmt"
+	"runtime"
 	"sync/atomic"
 	"time"
 
@@ -17,6 +19,8 @@ type SimpleCache[K comparable, V any] interface {
 	GetWithLoader(key K, loader func(key K) (V, time.Duration, error)) (V, error)
 	Keys() []K
 	Values() []V
+	Len() int
+	OnExpiration(fn func(K, V)) func()
 }
 
 type Options struct {
@@ -39,9 +43,17 @@ func NewSimpleCache[K comparable, V any](options ...Options) SimpleCache[K, V] {
 	}
 
 	c := ttlcache.New[K, V](opts...)
-	return &simpleCache[K, V]{
+	cache := &simpleCache[K, V]{
 		data: c,
 	}
+	go cache.data.Start()
+
+	// Automatic cleanup to prevent goroutine leak when cache is garbage collected
+	runtime.AddCleanup(cache, func(ttlCache *ttlcache.Cache[K, V]) {
+		ttlCache.Stop()
+	}, cache.data)
+
+	return cache
 }
 
 const evictionTimeout = 1 * time.Hour
@@ -127,3 +139,15 @@ func (c *simpleCache[K, V]) Values() []V {
 	})
 	return res
 }
+
+func (c *simpleCache[K, V]) Len() int {
+	return c.data.Len()
+}
+
+func (c *simpleCache[K, V]) OnExpiration(fn func(K, V)) func() {
+	return c.data.OnEviction(func(_ context.Context, reason ttlcache.EvictionReason, item *ttlcache.Item[K, V]) {
+		if reason == ttlcache.EvictionReasonExpired {
+			fn(item.Key(), item.Value())
+		}
+	})
+}